riddelliine and Neoplasms

The level of 5-hydroxymethylcytosine (5-hmC) converted by ten-eleven translocation (TET) family is decreased in cancers. However, whether 5-hmC level is perturbed in early stages of carcinogenesis caused by genotoxic carcinogens is not defined. 5-hmC levels and TET2 expression were measured in liver of rats treated with genotoxic carcinogens, riddelliine, or aristolochic acid. Levels of 5-hmC and TET2 expression decreased in the liver of the carcinogens-treated rats. Loss of 5-hmC correlates well with documented induction of genetic mutations by the carcinogens, suggesting that TET2-mediated 5-hydroxymethylation plays an epigenetic role in early state of carcinogenesis.

Topics: 5-Methylcytosine; Animals; Aristolochic Acids; Carcinogenesis; Carcinogens; Cytosine; DNA-Binding Proteins; Epigenesis, Genetic; Liver; Mutation; Neoplasms; Pyrrolizidine Alkaloids; Rats; Rats, Inbred F344; Rats, Transgenic

Pyrrolizidine alkaloid-containing plants are widespread in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Pyrrolizidine alkaloids are among the first chemical carcinogens identified in plants. Previously, we determined that metabolism of pyrrolizidine alkaloids in vivo and in vitro generated a common set of DNA adducts that are responsible for tumor induction. Using LC-ESI/MS/MS analysis, we previously determined that four DNA adducts (DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4) were formed in rats dosed with riddelliine, a tumorigenic pyrrolizidine alkaloid. Because of the lack of an adequate amount of authentic standards, the structures of DHP-dA-3 and DHP-dA-4 were not elucidated, and the structural assignment for DHP-dG-4 warranted further validation. In this study, we developed an improved synthetic methodology for these DNA adducts, enabling their full structural elucidation by mass spectrometry and NMR spectroscopy. We determined that DHP-dA-3 and DHP-dA-4 are a pair of epimers of 7-hydroxy-9-(deoxyadenosin-N(6)-yl) dehydrosupinidine, while DHP-dG-4 is 7-hydroxy-9-(deoxyguanosin-N(2)-yl)dehydrosupinidine, an epimer of DHP-dG-3. With the structures of these DNA adducts unequivocally elucidated, we conclude that cellular DNA preferentially binds dehydropyrrolizidine alkaloid, for example, dehydroriddelliine, at the C9 position of the necine base, rather than at the C7 position. We also determined that DHP-dA-3 and DHP-dA-4, as well as DHP-dG-3 and DHP-dG-4, are interconvertible. This study represents the first report with detailed structural assignments of the DNA adducts that are responsible for pyrrolizidine alkaloid tumor induction on the molecular level. A mechanism of tumor initiation by pyrrolizidine alkaloids is consequently fully determined.

Topics: Animals; Carcinogens; Cattle; Chromatography, High Pressure Liquid; DNA; DNA Adducts; Magnetic Resonance Spectroscopy; Microsomes, Liver; Neoplasms; Pyrrolizidine Alkaloids; Rats; Spectrometry, Mass, Electrospray Ionization; Stereoisomerism

Topics: Animals; Carcinogens; Humans; Neoplasms; Pyrrolizidine Alkaloids

Monocrotaline is a representative naturally occurring genotoxic pyrrolizidine alkaloid. Metabolism of monocrotaline by liver microsomes of F344 female rats generated (+/-)6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) and monocrotaline-N-oxide as major metabolites. Metabolism in the presence of triacetyleandomycin, a P450 3A enzyme inhibitor, reduced the formation of DHP by 52% and monocrotaline N-oxide formation by 59%. Dexamethasone significantly induced microsomal monocrotaline metabolizing enzyme activities in rat liver and lung. Previously, we have identified a set of DHP-derived DNA adducts from DHP-modified calf thymus DNA by (32)P-post labeling/HPLC analysis. Metabolism of monocrotaline in the presence of calf thymus DNA resulted in a similar set of DHP-DNA adducts. These DHP-DNA adducts were also found in the liver DNA of rats treated with monocrotaline. The time course of the DHP-derived DNA adduct formation and removal in the liver of rats gavaged with a single dose (10mg/kg) of monocrotaline was similar to that of rats treated with riddelliine. The levels of DHP-DNA adducts in liver DNA of rats treated with monocrotaline were much lower than that of riddelliine-treated rats. Results from this study indicate that (i) DHP is a common reactive metabolite for retronecine-type of pyrrolizidine alkaloids, (ii) the formation of DHP-derived DNA adducts in the liver DNA of rats treated with monocrotaline suggests that monocrotaline-induced tumorigenicity is through a genotoxic mechanism.

Topics: Animals; Dicarbethoxydihydrocollidine; DNA Adducts; Female; Liver; Monocrotaline; Neoplasms; Pyrrolizidine Alkaloids; Rats; Rats, Inbred F344